The general secretary of right-wing party Vox, Javier Ortega, gives a speech during the Andalucían regional election in Spain.
Photograph: Rafa Alcaide/EPA

The Vox party in Spain has become the first far-right group to triumph at the ballot box since the country’s return to democracy after the death of Francisco Franco in 1975.

Vox, a small but increasingly vocal party that opposes Catalan independence and has vowed to take a tough line on immigration and abortion, exceeded all predictions to take 12 seats in the Andalucían regional election on Sunday. It could hold the key to the formation of the next government of the populous southern region.

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Although the ruling Spanish socialist party (PSOE) won the elections, taking 33 of the 109 seats in the regional parliament, its support collapsed in the heartland it has ruled since 1982.

Even with the support of the Podemos-led Adelante Andalucía coalition – which won 17 seats – the PSOE would still be short of the 55 seats needed for a majority in the regional parliament.

The conservative People’s party (PP) took 26 seats, while the centre-right Ciudadanos party won 21. Were the two rightwing parties to join forces with Vox, they would jointly command a majority, with 59 seats.

The prime minister, Pedro Sánchez, said Vox’s breakthrough would not alter the government’s determination to press on with its agenda. “My government will carry on with its pro-European renovation project for Spain,” he tweeted. “The results in Andalucía strengthen our commitment to defending democracy and the constitution in the face of fear.”

The former Belgian prime minister Guy Verhofstadt, who leads the liberal ALDE group in the European parliament, congratulated Ciudadanos on its “huge gains” in the election, but added: “The success of the far right, however, should worry us all. We face a battle for Europe’s soul at the European elections in May.”

The PSOE’s regional leader, Susana Díaz, called Sunday “a sad night for the socialist party”. “There has been a real loss of ground for the left,” she said. “But the worst thing is that the extreme right, a phenomenon that has appeared in the rest of Europe, has arrived here.”

Diaz said she would call on other parties to “build a firewall against the extreme right in Spain”, adding: “Each party must decide if they are against the extreme right or if they will rely on their support to enter into government.”

Two members of Vox shouted insults at Diaz when she cast her ballot early in Seville early on Sunday. Vox said both people had been removed from their positions as voting monitors due to inappropriate behaviour.

“The Andalusians have made history … and got rid of 36 years of socialist rule,” said the Vox leader, Santiago Abascal.

Abascal said his party had achieved its win by listening to people in Andalucía who wanted a party that spoke about “issues that they felt and no other party represented”.

He dismissed criticism from mainstream parties and media, saying they were simply out of touch. “You haven’t understood anything,” Abascal told a news conference. “Every time you insult us, you are insulting the millions of Spanish people who listen to us and identify with our message.”

The party, which was formed by disenchanted former PP members, has capitalised on the Catalan independence crisis and on public concern at the record number of migrants and refugees arriving on Spanish shores this year.

“Now is the moment to say loud and clear who we are and that we have come to stay,” the Vox candidate Francisco Serrano told a crowd of supporters who chanted: “Spain! Spain! Spain!”